1. The Field of the Invention
The present invention relates to a light source, in particular a semiconductor laser diode, as well as to a method of manufacture thereof.
2. The Relevant Technology
A laser diode is a semiconductor diode, the active material of which, consisting of a p-i-n heterojunction, allows obtaining optical gain within a wavelength range determined by the epitaxial structure of the alloys of semiconductor materials it is composed of, and such heterojunction (optical medium) is inserted in a cavity that is resonant for the optical field.
The population inversion necessary for laser emission is obtained by injection of current into the junction itself. The terminal facets in a laser diode chip can be used for forming the resonant cavity (called laser cavity); an external mirror and/or a Bragg reflector may also be used for this purpose.
Laser diodes are extremely efficient sources of coherent light with high density of power and brilliance, and are widely used in opto-electronic devices.
The power emitted by semiconductor diodes may vary from a few tens or hundreds of milliwatt (information transmission applications) to values in excess of ten Watt (high-power diodes).
In order to obtain higher power ratings, e.g., for direct material processing applications, the space multiplexing technique is normally employed: the beams of the single laser diodes are simply overlapped in a well-defined spatial region. However, in such a way, the quality of the emitted beam (analytically defined by a known parameter called BPP, Beam Parameter Product) generated by such multiplexing is degraded in comparison with the quality of the original beams, because the size of the beam is increased.
Such degradation prevents the use of laser diodes organized in such a configuration for most material processing applications.
On the other hand, it is possible to sum up the power of the single beams without jeopardizing the quality of the final beam thus obtained, provided that the starting beams differ in some properties, e.g., polarization or wavelength. The latter case (referred to as spectral or wavelength multiplexing) is particularly interesting, in that it is possible to sum up N devices, the wavelength of which is defined as λ1 □ λN, in order to obtain a beam having the same qualities as the original optical beam, but N times more powerful.
Therefore, in order to allow direct use of high-power laser diodes for material processing applications without requiring a fiber or gas laser as a high-quality beam source, it is necessary to employ lasers having a narrow emission spectrum and a very stable emission. These aspects are crucial for being able to sum up the powers emitted by an array of laser diodes via wavelength multiplexing, while keeping the quality of the beam of each diode unaltered.
The technique which is most commonly used in order to stabilize the emission wavelength requires the use of external volumetric stabilizers (Bragg gratings on crystals or thin films).
One example is shown in document U.S. Pat. No. 9,209,605, which describes a laser diode wherein dichroic reflectors are arranged sequentially one after the other so as to form an array, and wherein each reflector has a sequential array index. Individual reflectors of the array re-direct sub-beams coming from individual laser emitters having the same array indices to propagate them through the dichroic reflectors having higher array indices, so as to form a combined optical beam.
That part of the optical power which is back-reflected by the dichroic reflectors serves the very purpose of stabilizing the emission of each laser diode.
Such an approach offers the advantage of allowing the use of Fabry-Perot laser diodes, which are relatively simple from a technological viewpoint, without any kind of stabilization integrated in the chip itself. However, this configuration requires a more complex and expensive module layout, because suitable reflectors need to be inserted in order to stabilize the emission (which would otherwise be as wide as several tens of nanometer, as well as extremely sensitive to the temperature and polarization current of the lasers themselves).
Moreover, the stability of the back-reflection of the mirror, which is necessary to obtain a stabilized emission without introducing high losses that would be critical for power modules, imposes very stringent requirements as regards the mechanical characteristics of the module itself and the optical characteristics of the focusing systems, resulting in high production costs poorly scalable in volume productions.
Document “10 W-reliable 90 μm-wide broad area lasers with internal grating stabilization”, P. Crump*, J. Fricke, C. M. Schultz, H. Wenzel, S. Knigge, O. Brox, A. MaaSdorf, F. Bugge, G. Erbert describes the manufacture of power laser diodes with a narrow spectral emission that is stable as temperature changes. In a first example, grating surfaces are made by etching in order to form the rear reflector as a distributed Bragg grating (DBR). In this way, the rear facet is replaced with a wavelength-selective mirror, thereby obtaining a stable and narrow emission.
In a second example, the grating is inserted into the semiconductor by using etching and growth techniques in order to form distributed DFB lasers, wherein the rear facet has a high-reflectivity coating and the DFB operates as a low-reflectivity coupler. This latter solution, which is simpler in terms of grating construction, offers however lower power output and wavelength stabilization.
Document “High-Brilliance Diode Lasers with Monolithically-Integrated Surface Gratings as Sources for Spectral Beam Combining”, J. Fricke*, P. Crump, J. Decker, H. Wenzel, A. MaaSdorf, G. Erbert, G. Trankle describes an example of application of the above-described wavelength stabilization techniques for obtaining increased power with constantly good beam quality by means of a spectral combination. Stabilization of the wavelengths of the single emitters is achieved by monolithic integration of a Bragg grating in the chip.
With the exception of the integration of the Bragg mirror, the above-described solutions are still traditional as far as the other technological steps are concerned, since the laser cavity must still be created by cleavage (fracturing along the crystallographic axis) of the process wafer, and the laser diodes thus obtained still need suitable depositions of dielectric material on their facets (typically at bar level) for the dual purpose of passivating and preserving the facet itself and modify (towards the upward-reflected or downward-reflected, according to the case) the natural reflectivity at the semiconductor-air interface.